For which of the following elements do ground-state atoms contain electrons in d Orbitals? A) Fluorine, B) phosphorus, C) iodine, D) aluminum, E) silicon.
In this question, we’re being asked about ground-state atoms. In a ground-state atom, all the electrons that the atom contains are in the lowest possible energy levels. In an atom, electrons occupy regions of space called orbitals, which are found in the various shells or energy levels outside of the nucleus. According to the aufbau principle, in the ground-state of an atom, electrons fill orbitals of the lowest available energy level before occupying higher energy levels. The German word aufbau translates as to build up. And just as you would build an apartment block from the ground up, electrons fill the lowest energy levels first. That is, the shells closest to the nucleus.
In each energy level, there are regions of space where we are likely to find electrons. These known as orbitals. In the diagram on the left here, a set of p orbitals in three-dimensional space is shown. Do not confuse the word orbital with an orbit. In energy level one, we encounter s orbitals only. These are spherical regions of space. In energy level two, we encounter s and p orbitals. Energy level three can contain s, p, and d orbitals. In energy level four, we may find s, p, d, and f orbitals. So in every energy level, we have a set of s orbitals and these can contain two electrons. Electrons are shown paired in the square boxes. There are three sets of p orbitals in energy level two and above. Each p orbital can contain up to two electrons. This gives us a total of six electrons, when those p orbitals are fully occupied.
There are five sets of d orbitals. When fully occupied, these can accommodate up to 10 electrons. And in energy level four and above, there are seven sets of f orbitals. These can accommodate up to 14 electrons when full. Because there is an overlap in the energy levels of some of these orbitals, the order in which they feel is not quite as straightforward as it may seem. The simplest way to remember the order of filing is to follow the blue line shown in the diagram diagonally from top to bottom. If we do this for the first 36 electrons, we get the order shown here. Notice that the four s subshell fills up before the three d subshell because it is slightly lower in energy. This is, in fact, the full electron configuration for krypton, which has atomic number 36.
If we look at the atomic number for each element in our list, we can find out how many electrons we are dealing with for each element. Fluorine, phosphorus, aluminum, and silicon are all found within the first three periods of the periodic table. It’s relatively straightforward to write out their electronic configurations. It becomes immediately obvious that the highest occupied orbitals for these four elements are p orbitals. They are, therefore, referred to has p block elements. Notice that the question is asking us to identify an element from the list, which contains d orbitals that are occupied by electrons. Fluorine, phosphorus, aluminum, and silicon do not contain occupied d orbitals. They are, therefore, not correct answers to this question.
If we use the periodic table like a map, it’s quite easy to see how each subshell is filled as we move from left to right and top to bottom of the periodic table. Having identified that fluorine, phosphorus, aluminum, and silicon do not contain occupied d orbitals, we now need to check where iodine lies in our periodic table. Iodine is a halogen in group 17 or seven of the periodic table. A quick check of its electronic configuration, which contains 53 electrons, reveals that it does contain occupied d orbitals. The configuration shown here shows the order of filling. You may see this written in order of energy levels. So the ground-state electronic configuration of iodine does contain occupied d orbitals. And iodine is the correct answer.